Method of forming a fibrous web

ABSTRACT

In a method of forming a layered fibrous web, the method includes the steps of leading at least two different fiber suspensions through a head box ( 10 ), separating the fiber suspensions from each other by a blade device ( 18, 180 ), supplying the fiber suspensions to a web forming device ( 24 ) and forming a water layer having a transversely uniform flow at an outlet at the end of the blade device ( 18, 180 ), such that the water layer constitutes an extension of the blade device and thereby prevents mixing of the fiber suspensions. The blade device ( 18, 180 ) can be freely movable in a substantially vertical direction in order to permit an equalization of pressure between the fiber suspension layers separated by the blade device and thereby reducing wake effects downstream the edge of the blade device. A paper machine for carrying out the method is also disclosed.

This application is a 371 of PCT/SE02/02217 filed on 3 Dec. 2002.

TECHNICAL FIELD

The present invention relates to a method of forming a layered fibrousweb and a machine therefor.

DESCRIPTION OF THE BACKGROUND ART

In the paper industry, it is often desirable to form multi-layeredfibrous pulp webs, where different fiber suspensions are used in thedifferent layers. A paper can for example consist of a core made ofrecycled fiber material while the surface layers consist of a differentfiber material in order to give the paper a desired surface. Suchmulti-layered webs can be produced by means of layered head boxes, wherethe different fiber suspensions are separated by means of at least oneblade. At the outlet of the head box, the fiber suspension layers aredispensed on top of each other on a paper forming wire and therebyforming a multi-layered fibrous pulp web. Since the different layersoften consist of materials with different fiber properties or evendifferent colors, it is essential to prevent mixing of the layers. Themixing is mainly caused by the wake that is formed behind the trailingedge of the blade. These wakes generate strong turbulent mixing, whicheffectively prevents the possibility to obtain a good layer purity.Several attempts to solve this problem have been made.

The British patent application GB, A, 2107751 discloses a paper machineand a process for producing a multi-layer fibrous pulp web, where twolayers of fiber suspension are separated by an intermediate flow ductthrough which water is flowing and thereby forming a separating layerbetween the fiber suspension layers. Said flow duct extends from theinlet end of the head box up to approximately the center of the head boxand consists of rigid partition walls. This construction requires asymmetrical position of the blades in the head box in order to achieveequal pressure on both sides of the blade at the trailing edge. Also,severe mixing will take place unless the partition walls forming theflow duct are extremely accurately positioned, which in reality ispractically impossible. As a consequence of this, it is not possible tofeed different volume flows of fiber suspension separated by thepartition walls without causing an unwanted difference in pressurebetween the fiber suspension layers. The positioning of the partitionwalls in order to achieve equal pressure on both sides of the blade mustfurthermore be adapted for one specific speed of the fiber suspension,which makes the paper machine inflexible to use.

SUMMARY OF THE INVENTION

The object of the invention is to solve the problems related to theprior art. This is achieved by means of a method according to claim 1.More specific, the present invention relates to a method of forming alayered fibrous web, comprising leading at least two different fibersuspensions through a head box, separating the fiber suspensions fromeach other by a blade device, supplying the fiber suspensions to a webforming device and forming a water layer having a transversely uniformflow at an outlet at the end of the blade device, such that the waterlayer constitutes an extension of the blade and thereby prevents mixingof the fiber suspensions. The method furthermore comprises arranging theblade device to be freely movable in a substantially vertical directionin order to permit an equalization of pressure between the fibersuspension layers separated by the blade device and thereby reducingwake effects downstream the edge of the blade device.

Furthermore, the invention relates to a paper machine including a headbox comprising at least one fiber suspension separating blade deviceprovided with a water outlet, the outlet being arranged so as to form awater layer which constitutes an extension of the blade device andhaving a transversely uniform flow rate. The blade device is arranged tobe freely movable in a substantially vertical direction in order topermit an equalization of pressure between the fiber suspension layersseparated by the blade device and thereby reducing wake effectsdownstream the edge of the blade device.

The blade device is hereby automatically adjusted to compensate fordifferences in pressure and consequently differences in speed of thedifferent fiber suspension layers. It is therefore possible to feeddifferent flow volumes of fiber suspensions in each layer in amulti-layered head box and keeping the speed of the useful e.g. whenforming a layered paper web with layers having different thickness. Itis also possible to position one or more blades asymmetrically in thehead box.

According to one preferred embodiment of the invention, water is pumpedthrough the blade device in internal channels in order to obtain apressure drop, which will assure that the water flow rate is the same inall channels. The channels terminate preferably at a distance from thewater outlet at the end of the blade device in order to provide auniform flow rate of the water coming out from the blade device.

In another preferred embodiment of the invention, a pressure drop screenis arranged inside the blade device transversely to the direction of theintended water flow. The pressure drop screen can preferably beconstituted of a porous material such as a net member arranged to spanthe entire width and height of a cross section of the blade device inorder to provide a uniform water flow out from the blade device.

The height of the water outlet from the blade device, and thereby theheight of the water layer is preferably kept constant, and the flow rateof the water is adjusted by changing the pumping pressure of the water.The water can be given a speed that is either higher or lower than thespeed of the fiber suspension. At a water speed lower than the fibersuspension speed, a wake effect is still present downstream the end ofthe blade device. A water speed that is higher than the fiber suspensionspeed can on the other hand result in a jet effect, which leads to anincreased mixing of the layers. By changing the flow rate of the waterit is possible to control the layering of the different fibersuspensions in order to achieve an optimal state, where minimal mixingbetween the layers at minimal water consumption is achieved.

The water is supplied to the blade device preferably by means of a feedpipe, which is arranged at an upstream end of the blade device andperpendicular to the direction of the intended water flow.

Pressure equalization between the fiber suspension layers that areseparated by the blade device can be achieved in different ways. In oneembodiment of the invention, the blade device is movable in such a waythat it is elastically deformable and thereby permitting theequalization of pressure between the fiber suspension layers.

In another preferred embodiment, the blade device is pivotally supportedat an upstream end thereof and can pivot freely around a pivot axisperpendicular to the intended flow direction, thereby permittingequalization of pressure between the fiber suspension layers. The waterfeed pipe is preferably connected to the blade device in a rotationallyfixed manner in such a way that the feed pipe coincides with the pivotaxis of the blade.

The blade device can furthermore be both elastically flexible andpivotally supported.

The blade device can comprise a pair of parallel mutually spaced sheets,arranged above each other and substantially parallel to the direction ofthe intended water flow. Alternatively, the blade device can comprise apair of sheets, which are arranged above each other and converge towardseach other in the direction of the intended water flow, whereby theoutlet from the blade device is narrower than the inlet.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a head box in accordance with the invention having aseparating water dispensing blade device.

FIG. 2 shows a cross section of the blade device taken along the 2-2line as shown in FIG. 1.

FIG. 3 shows a pivotally supported blade device.

FIG. 4 shows an alternative embodiment of the blade device.

FIG. 5 shows a head box in accordance with the invention having twoblade device.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

FIG. 1 shows a cross section through a head box 10 for a paper machine.The head box 10 is supplied with two different streams of fibersuspension via inlet nozzles 12, 14 that feed the fiber suspension intoducts 15, 16, which are arranged above each other and separated by meansof a substantially horizontal blade device 18. The fiber suspension isdispensed out from the head box 10 through an outlet 22 onto a paperforming wire 24, where the two layers of fiber suspensions aresuperposed to form a layered paper web. FIG. 2 shows a blade device 18constituted by an upper sheet 26, a lower sheet 28 and internal channelsin the form of parallel pipes 30 arranged between the upper and thelower sheet. The pipes 30 are arranged side-by-side and parallel to theflow direction of the fiber suspension. The sheets 26, 28, are arrangedparallel and mutually spaced as shown in FIGS. 1 and 3. Water is pumpedfrom an upstream end of the head box 10 by means of a pumpingarrangement (not shown), whereby the internal channels have a pressuredrop effect on the water flowing in the blade device (18). A uniformwater layer is hereby formed downstream of the end of the blade device18 forming an extension of the same. The water layer prevents or reducesthe occurrence of a wake downstream the edge of the blade device 18. Awake would cause development of vortices in the fiber suspension layersand thereby give rise to unwanted mixing of the layers. The blade device18 extends substantially from the up-stream end of the head box 10 tothe outlet 22. The pipes 30 terminate at a distance from the outlet endof the blade device 18 as shown in FIG. 3, in order to achieve a uniformwater flow out from the blade device 18.

An alternative embodiment of the invention is shown in FIG. 4, whereinthe blade device 180 comprises an upper sheet 260 and a lower sheet 280,which sheets converge towards each other in the direction of theintended water flow.

In another embodiment of the invention, the blade device 18, 180 isarranged without internal channels and is instead provided with one ormore pressure drop screens 38 arranged inside the blade device 18, 180transversely to the intended direction of the water flow as shown inFIG. 4. The pressure drop screen preferably comprises a porous materialsuch as a net and is arranged to span the entire width and height of across section of the blade device 18, 180 in order to provide a uniformwater flow out from the blade device 18, 180.

According to a preferred embodiment, the blade device 18, 180 isarranged to pivot freely around an axis 20, which is substantiallyperpendicular to the direction of the intended water flow. The bladedevice 18, 180 can be either rigid or elastically deformable. However,the height of the water outlet from the blade device 18, 180 and therebythe height of the water layer, is kept constant. In the embodimentcomprising elastically deformable blade device, this can be achievede.g. by using a rigid material for manufacturing the outlet part of theblade device 18, 180, while the rest of the blade device is manufacturedfrom an elastically deformable material.

The water speed can be controlled by means of adjusting the pumpingpressure of the water. A feed pipe 32, supplying water to the blade, isarranged in a position that coincides with the pivot axis 20. The feedpipe 32 is provided with openings 34 through which water is supplied tothe blade device 18, 180. The blade device 18, 180 is attached to thefeed pipe 32 in a rotationally fixed manner. The feed pipe 32 isrotationally arranged, preferably by means of a swivel connection 36 asshown in FIG. 3, so that the blade device 18, 180 can pivot freely.

According to an alternative embodiment, water can be fed to the bladedevice 18, 180 by means of a feed pipe having a single slotted,lengthwise aperture (not shown) through which water is supplied to theblade device 18, 180.

In another embodiment of the invention, the blade device 18, 180 iselastically deformable and has a non-pivotable mounting in the head box10. The pressure equalization between the fiber suspension layers ishereby achieved only by deforming movements of the blade device 18, 180.A feed pipe 32, supplying water to the blade, is arranged at theupstream end of the blade device, perpendicular to the direction of theintended water flow and provided with openings 34 through which thewater is supplied to the blade device 18, 180

The head box 10 can furthermore be provided with two or more bladedevice 18, 180, as shown in FIG. 5, whereby a multi-layered paper web isformed. Due to the self-adjustment of the blade device 18, 180 forpressure equalization layers, the vertical placement of the blade device18, 180 in the head box 10 can be either symmetric or asymmetric.

The invention is not limited to the exemplified embodiments describedabove. A person skilled in the art will find further applications withinthe scope of the invention in the accompanying claims.

1. A method of forming a layered fibrous web comprising the steps of:(a) leading at least two different fiber suspensions through a head box;(b) arranging at least one blade device to separate the at least twofiber suspensions from each other and to be freely movable in asubstantially vertical direction in order to permit an equalization ofpressure between the at least two fiber suspension layers separated bythe at least one blade device and thereby reducing wake effectsdownstream of an outlet end of the at least one blade device; (c)forming at least one layer of water having a transversely uniform flowat the outlet end of the at least one blade device, such that the atleast one layer of water constitutes an extension of the at least oneblade device to separate and thereby prevent mixing of the at least twofiber suspensions; and (d) supplying the at least two fiber suspensionswith the water therebetween to a web forming device.
 2. The methodaccording to claim 1, wherein the forming step includes pumping waterthrough the at least one blade device in internal channels.
 3. Themethod according to claim 2, wherein the internal channels terminate ata distance from the outlet end of the at least one blade device in orderto provide a uniform flow rate of the water coming out from the at leastone blade device.
 4. The method according to claim 1, wherein the heightof the outlet end of the at least one blade device is constant wherebythe thickness of the at least one layer of water is constant and thetransversely uniform flow has an adjustable flow rate by changingpumping pressure of the water.
 5. The method according to claim 1,further including the step of supplying water to an upstream end of theat least one blade device by a feed pipe arranged perpendicular to adirection of intended water flow.
 6. The method according to claim 1,further including the step of providing a pressure drop screen insidethe at least one blade device transversely to a direction of water flow.7. The method according to claim 1, wherein the arranging step includesarranging the at least one blade device to be movable in such a way thatthe at least one blade is elastically deformable, thereby permitting theequalization of pressure between the at least two fiber suspensionlayers.
 8. The method according to claim 1, wherein the arranging stepincludes arranging the at least one blade device to be movable in suchway that the at least one blade is pivotally supported at an upstreamend thereof and can pivot freely around a pivot axis perpendicular to aflow direction of the water, thereby permitting the equalization ofpressure between the at least two fiber suspension layers.
 9. The methodaccording to claim 8, wherein a feed pipe is connected to the at leastone blade device in a rotationally fixed manner such that the feed pipecoincides with the pivot axis of the at least one blade device.
 10. Themethod according to claim 1, wherein the transversely uniform flow ofthe at least one layer of water is adjustable with a speed independentof the speed of the at least two fiber suspension layers.